Generally, the present invention relates to a pressure washer which pumps liquid from an external source and supplies it to a spray nozzle at high pressure, typically even higher than 1,000 psi. More specifically, the invention relates to a bypass system, disposed between the liquid outlet and the liquid inlet of the pump of the pressure washer. The bypass system serves to regulate or relieve the pressure at the liquid outlet of the pump, both when the pressure washer is operating and the pump is pumping and when the pump is turned off. The system of the present invention prevents over-pressurization of the liquid at the liquid outlet and avoids the possibility that an undesirable initial burst of pressurized liquid will shoot through the spray nozzle, should the spray nozzle be turned on while the pump is off.
The pressure washer of the present invention may be embodied in a standing or portable version. In the standing version, a pumping section of the washer stands on the floor and has an elongated hose leading to a hand-held spray nozzle. The pathway through which the liquid is pumped to the spray nozzle is selectively openable to permit the liquid to be sprayed from the spray nozzle and closable to halt the spray of liquid. The portable version is, on the other hand, comprised of a single, portable, hand-held unit combining the nozzle with the pumping unit.
Some pumps are designed to operate only when liquid spraying is required. In the standing form of the pressure washer, on the other hand, the pump is typically operated continuously whether the liquid pathway to the spray nozzle is open or closed. Consequently, the standing pressure washer requires protection of the pump from overheating and other effects when the liquid pathway which is controlled by the spray nozzle is closed while the pumping action continues. Another type of protection is against excess pressure in the system downstream of the pump, due, for instance, to a blockage. One known technique for protecting the continuous pumping washer system comprises selective bypassing of pumped liquid from the pump outlet back to the pump inlet when the liquid pathway is closed. A valve controls the bypass arrangement to permit bypass recirculation at a lower pressure to prevent overheating of the pump elements.
However, even in a pressure washer in which the pump is turned off when liquid spraying is halted, a problem exists in conventional pumps in that the pump motor shuts down gradually, not instantly. As a result, with the pump motor off and the spray nozzle closed, the gradual stopping of the pump motor has the effect of pressurizing the output chamber of the pressure washer, with the undesirable result that when the spray nozzle is actuated while the pump is off, there is an initial burst of highly pressurized liquid through the nozzle.
Further, often the pressure washer is used to pump liquid, particularly water at high pressure. When the water is used for cleaning purposes, it is sometimes desired to mix with it another liquid, for example, a detergent, a chemical, or the like. Appropriate mixing means are then needed for controllably mixing the additional liquid with the water being pumped. Various such mixing means are known in the art, but conventionally these known mixing devices have been provided separately and independently of the aforementioned bypass system. This complicates the construction and adds to the cost of pressure washers.
Many known pressure washers use piston/cylinder pumps, in which the piston is reciprocated by various means. It is further known and preferred to provide multi-piston pumps to optimize the balance, speed, torque, bearing life, valve design, flow rate, efficiency, and the spray characteristics, e.g. spray continuity and uniformity, of the pressure washer. In one multi-piston pump, the pistons are driven in a manner wherein they tend to wobble or swash as they reciprocate within their cylinders and move past a stationary resilient seal. It is therefore essential to provide a piston sealing arrangement which can withstand the wobbling/swashing of the pistons while still providing good sealing and a simple and easy to service piston sealing construction.